is it just me or did he get a hell of a lot harder in challenge mode

Yes, he did. But he's easy, upgrade to Titan Deadeye Mine V4, get the PDA, and just keep blasting him.

Or just get the RYNO.

i just kept circling him wit shield and ryno and hive bee mine and pratically all my wepons

(offtopic) but it took me ages to do the clank flying through space thing on challenge

oh the ryno you say? i thought id try getting that and challenging him. im at 6.5mil and counting...... shouldnt have bought titan dual lacerators, shield thing and shock rocket..... ah well

Im on him the first time and I might try using the fire-bomb aurmor and a v5 lazer tracer (I dont have It v5 yet so I dont know the new name)

use lacers to get shield gone then laser takes his health down almost instantly

I would say get the Optical Maser Array and Electro Rocket to Titan V4, then use the Optical Maser to take down his shield in half a heartbeat and the electros to damage him. Once you run out of ammo, just use the PDA. (NOTE the rockets would be a LOT better with the aftershock mod).
The Titan V4 Repulsor Field would be nice too, but if you need all three you might as well save up for the RYNO as well. If you can survive without the shields for most of the fight, then buy the first 2.

just to let you know you get all ammo back when you face him

Okay, I've changed my mind.
I just did the entire Challenge Mode, including Otto, with only 2 Titan weapons - the Lacerators and the O.M.A.
Just use the PDA, use the O.M.A. to take off his shield AND shoot him when he's running away.
Then when he stays still and doesn't put up a shield, just put a (normal) Repulsor Field on, stand still, and fire the Maser until he's dead (buying ammo when necessary with the PDA).