Aston Villa striker Gabriel Agbonlahor has been told his best is yet to come as he prepares to turn his sights on Portsmouth on Sunday.[LNB] Agbonlahor has recently hit the 50-goal mark in his Villa career but manager Martin O'Neill has challenged the England international to move his game on to the next level.[LNB]O'Neill said: "What he has got to do is continue this improvement. He is not the finished article and he can improve his finishing.[LNB]"I think you can always feel that Gabby will score his fair quota of goals.[LNB]"But I think even he believes that he can improve by 50% more in the future.[LNB]"Goals can speak volumes for players. He can do other things for us but over-all his game has improved out of all recognition."[LNB]Agbonlahor has scored 15 goals so far this season and a flurry in the final five games of the season could be crucial as Villa bid to qualify for Europe next term.[LNB]Villa are outsiders for a fourth place finish as they currently trail Manchester City by five points and a challenge for a Europa League spot is more likely.[LNB]O'Neill added: "People here at Villa have talked about fourth spot for a long time.[LNB]"Manchester City have spent in the region of £200million and for £200million there is a fair chance you are going to get some really decent players and that is exactly what they have done.[LNB]"Harry Redknapp summed it up five or six weeks ago when he was asked about Tottenham Hotspur.[LNB]"He said that they could finish fourth or finish seventh and that was before Everton came into the picture.[LNB]"So these teams are vying for the same things as Villa and no-one has said it would be easy.[LNB]"In fact it has been exceptionally difficult.[LNB]But it has been worth the trip.[LNB]"I felt at the start of the season when someone asked me if we could finish in the same position as last year (sixth) that it would be exceptionally difficult because we have to be contesting every single week, every single month."[LNB]But O'Neill knows that things could have been a lot worse this season.[LNB]Villa lost their opening game at home to Wigan and crashed out of the Europa League at the hands of Rapid Vienna.[LNB]"Losing our opening day game against Wigan was probably the lowest point of the season as far as I am concerned and that includes the 7-1 defeat at Chelsea," said O'Neill.[LNB]"But we fought back and have not been out of the top seven since August.[LNB]"We have also played a Carling Cup Final and contested the FA Cup semi-final.[LNB]"So I've been absolutely delighted with the team and with five games to go there are things still be played for.[LNB]"What we have tried to do over the last couple of seasons is step up another notch and the side has improved immensely.[LNB]"We lost a major player this season in Gareth Barry and that was a blow to us.[LNB]"But I know Gareth wanted to go and sample football elsewhere and that is fine."